GENEVA, May 18 (Xinhua) -- The head of the World Health Organization (WHO) on Monday cautioned the world again on the threat of a possible influenza pandemic as the new A/H1N1 flu virus continues to spread in more and more countries.

"The new H1N1 virus spreads very easily from person to person, spreads rapidly within a country once it establishes itself, and is spreading rapidly to new countries. We expect this pattern to continue," Dr. Margaret Chan told the 62nd World Health Assembly (WHA).

The emergence of the new virus creates great pressure on governments, ministries of health, and the WHO to make the right decisions and take the right actions at a time of great scientific uncertainty, she said.

Chan recognized that the H1N1 virus "presently causes mainly mild illness, with few deaths, outside the outbreak in Mexico."

But she warned that the behavior of influenza viruses is notoriously unpredictable, and the behavior of pandemics is as unpredictable as the viruses that cause them.

"This virus may have given us a grace period, but we do not know how long this grace period will last. No one can say whether this is just the calm before the storm," she said.

She noted that the presence of the new virus has now been confirmed in several countries in the southern hemisphere, where epidemics of seasonal influenza will soon be picking up.

"We have every reason to be concerned about interactions of the new H1N1 virus with other viruses that are currently circulating in humans," she said.

Moreover, no one can say how the H5N1 avian influenza virus, which is now firmly established in poultry in several countries, will behave when pressured by large numbers of people infected with the new H1N1 virus, she added.

According to the WHO's latest tally, the new flu virus has caused 8,829 infections and 74 deaths in 40 countries and regions.

The WHA is the supreme decision-making body of the WHO. It is usually held in Geneva in May and participated by health ministers of member states.

This year's assembly was expected to run through May 22, five days shorter than originally planned because health ministers are busy handling the disease situation in their own countries and need to go back earlier.